Edit: here's a handy infographic: https://www.prisonpolicy.org/reports/pie2016.html
Note that while only a small proportion of the overall prison population is in federal prison, a much larger proportion of drug convicts are in federal prison.
"There's about 95,000 people in federal prison with a drug offense as their most serious charge. About 11,500 of those charges involved primarily marijuana. (The overwhelming majority of that is for trafficking, not possession.)"
So no, your statement is not correct. If we take 'overwhelming majority' of 11,500 as 60%, that would leave ~5000 prisoners whose 'most serious charge' is possession of marijuana. Or about 2.5% of the federal inmate population (all this assuming that the figures quoted are correct, I didn't source them myself).
There's about 95,000 people in federal prison with a drug offense as their most serious charge. About 11,500 of those charges involved primarily marijuana. (The overwhelming majority of that is for trafficking, not possession.)
Rolling Stone estimated about 40,000 people in prison for convictions primarily involving marijuana, with about half of those involving marijuana alone: http://www.rollingstone.com/culture/lists/top-10-marijuana-m....
That is of course not to say that justice, for a few tens of thousands of people isn't an important thing! Though I hesitate a little bit to put trafficking in the same "justice" category as using. It's generally accepted that the government has much more leeway to control what is sold in the market than to control what people do with their bodies.